U.S. patent number 7,963,981 [Application Number 10/826,285] was granted by the patent office on 2011-06-21 for bone fixation plate.
This patent grant is currently assigned to Globus Medical, Inc.. Invention is credited to Lawrence Binder, Matthew Hansen.
United States Patent |
7,963,981 |
Binder , et al. |
June 21, 2011 |
Bone fixation plate
Abstract
An apparatus for reducing the profile of a bone fixation plate
while preventing backing out of screws is disclosed. The apparatus
includes at least one section of relief and sections of engagement.
The plate has at least two openings though which two screws can
pass through bony tissue. As the screw is tightened, it will begin
to lag the plate to the bone. When the screw head interferes with
the plate at the interference point, there is a slight resistance
that insertion forces can overcome. When the screw is advanced
further, it snaps into the sliding fit area and is allowed to move
freely. The forces that cause the screw to back out from the plate
are preferably not strong enough to pass the screw head back past
the interference section. It may be desirable to include a set
screw to help prevent backout.
Inventors: |
Binder; Lawrence (Doylestown,
PA), Hansen; Matthew (Schwenksvilie, PA) |
Assignee: |
Globus Medical, Inc. (Audubon,
PA)
|
Family
ID: |
35097247 |
Appl.
No.: |
10/826,285 |
Filed: |
April 19, 2004 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20050234455 A1 |
Oct 20, 2005 |
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Current U.S.
Class: |
606/289 |
Current CPC
Class: |
A61B
17/8033 (20130101); A61B 17/8047 (20130101); A61B
17/861 (20130101); A61B 17/8042 (20130101); A61B
17/844 (20130101); A61B 17/8052 (20130101); A61B
17/8038 (20130101); A61B 2017/8655 (20130101); A61B
17/1728 (20130101) |
Current International
Class: |
A61B
17/80 (20060101) |
Field of
Search: |
;606/69-71,280-291 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
ISR PCT/US05/13253. cited by other .
Writ.Opn. PCT/US05/13253. cited by other .
Haid et al., "The Cervical Spine Study Group anterior cervical
plate nomenclature," Neurosurg. Focus/vol. 12, Jan. 2002. cited by
other.
|
Primary Examiner: Barrett; Thomas C
Assistant Examiner: Araj; Michael J
Claims
The invention claimed is:
1. An apparatus for fixing a plate to bony material, comprising: a
plate having a unitary body with at least one opening having a
spherical curvature extending at least partially through the
thickness of the plate; and at least one fastener having a head
that interferes with an interference point of the plate; wherein
the head is capable of engaging with and passing the interference
point to communicate with the spherical curvature, wherein the
interference point is a portion of the unitary body of the plate
and conforms to the spherical curvature of the at least one opening
and located at an upper portion of the at least one opening,
wherein the interference point includes a plurality of relief areas
and a plurality of engagement areas, and wherein the fastener head
comprises a partially spherical outer surface corresponding
approximately to the spherical surface of the plate opening, at
least one slit located on the fastener head to permit outward
expansion of the fastener head, and a locking screw capable of
being received in a receptacle formed in the fastener head.
2. The apparatus according to claim 1, wherein the plurality of
relief areas are diametrically opposed.
3. The apparatus according to claim 2, wherein the at least one
relief area comprises less than about 40% of the interference
point.
4. The apparatus according to claim 2, wherein the at least one
relief area comprises less than about 30% of the interference
point.
5. The apparatus according to claim 1, wherein at least two
tangents from an outer most portion of the spherical curvature of
the plate intersect.
6. The apparatus according to claim 5, wherein an angle of
intersection of the at least two tangents is between about 1 and
about 5 degrees.
7. The apparatus according to claim 5, wherein an angle of
intersection of the tangents is between about 1 and about 3
degrees.
8. An apparatus for fixing a plate to bony material, comprising: a
plate having a unitary body and comprising at least one opening
having a spherical curvature; and at least one fastener having a
head capable of engaging with and passing through a interference
point of this spherical curvature; wherein the fastener is
prevented from backing out of the opening by the interference
point, wherein the interference point is a portion of the unitary
body and conforms to the spherical curvature of the at least one
opening and located at an upper portion of the at least one
opening, wherein the interference point includes a plurality of
relief areas and a plurality of engagement areas, and wherein the
fastener head comprises a partially spherical outer surface
corresponding approximately to the spherical curvature of the plate
opening, at least one slit located on the fastener head to permit
outward expansion of the fastener head, and a locking screw capable
of being received in a receptacle formed in the fastener head.
9. The apparatus according to claim 8, further comprising another
opening selectively positioned to increase interference at the
interference point.
10. The apparatus according to claim 8, wherein at least two
tangents to the spherical curvature intersect.
Description
FIELD OF THE INVENTION
The present invention relates to a bone fixation plate used to
stabilize vertebrae and other bony anatomy. More specifically, the
present invention relates to a cervical plate having a minimized
profile that easily and reliably prevents backout of fastening
devices.
BACKGROUND OF THE INVENTION
Bones and bony structures are susceptible to a variety of
weaknesses that can affect their ability to provide support and
structure. Weaknesses in bony structures may have many causes,
including degenerative diseases, tumors, fractures, and
dislocations. Advances in medicine and engineering have provided
doctors with a plurality of devices and techniques for alleviating
or curing these weaknesses.
The cervical spine has presented the most challenges for doctors,
partially due to the small size of the vertebrae and the spacing
between adjacent vertebrae. Typically, weaknesses in the cervical
spine are corrected by using devices that fuse one or more
vertebrae together. Common devices involve plate systems that align
and maintain adjacent cervical vertebrae in a desired position,
with a desired spacing.
These devices, commonly referred to as bone fixation plating
systems, typically include one or more plates and screws for
aligning and holding vertebrae in a fixed position with respect to
one another. Initial devices used stainless steel plates and
screws. In order to remain fixed in place, the screws were required
to pass completely through the vertebrae and into the spinal canal.
These devices caused many complications and involved significant
risks. To allow a screw to pass, drilling and then tapping of the
vertebrae was required. In the process, instruments came within
close proximity of the spinal cord, which required extreme care on
the part of the surgeon.
In addition to the risks of surgically applying bone fixation
plates, other complications arose. Commonly, these problems involve
loosening and failure of the hardware. Two common failures are the
breakage of the plates, and the backing out of the screws into soft
tissues of the patient's body. The backing out of the screws is
typically a result of the screws failure to achieve a sufficient
purchase in the bone, although the stripping of the screws has also
been known to cause this problem. Regardless of the cause of the
hardware failures, a surgeon must repair or replace the broken
parts, which requires undesirable invasive procedures.
Advances in material science allowed engineers to manufacture bone
fixation plates out of materials that would resist breakdown within
a body. However, the backing out of screws remained a problem. Many
solutions were devised in an attempt to prevent this from
occurring. One prevalent solution involved minimizing the length of
the screw in order to prevent screw to plate junction breakage of
the screw. However, the shortened screw is typically unable to
achieve a sufficient purchase in the bone. Shortened screws often
provide very little holding power and inadequate tactile feedback
to the surgeon. Tactile feedback to the surgeon is important to
signal completion of tightening prior to stripping of the screw
within the bone.
An alternate solution involves increasing the length of the screws
in order to achieve sufficient purchase to hold the plate in place.
While the use of longer screws can provide bicortical fixation,
this method also has its drawbacks. Primarily, long screws increase
the chances of interference with each other when they are screwed
into bony tissue at an angle. In addition, many bone fixation
plating systems place bone grafts between vertebrae. The bone
grafts are eventually supposed to spur the growth of bone between
the vertebrae, so that the vertebrae become fused together
naturally.
In order for this to occur, the bone fixation plating needs to
maintain a desired spacing between the vertebrae, which is filled
by the bone grafts. However, it is common for the bone grafts to
experience compression, which separates at least one of the
adjacent vertebrae from the bone graft. Cervical plates that employ
long screws do not allow for sufficient movement of the vertebrae
to accommodate the compression of the bone graft, because the
purchase of the screws is too great. Thus, the vertebrae cannot
move and are unable to adjusting to the compression of the bone
graft.
Another method of preventing the backing out of screws involves
placing a second plate over the screws. This second plate functions
to interlock the screws, preventing them from backing out. However,
this method of securing screws often becomes bulky, resulting in a
large and undesirable profile. In addition, these configurations
require carrying out multiple steps or using a multi-piece assembly
in order to block an opening through which a loose fastener head
may pass. For instance, the use of a c-ring that can expand as the
fastener head is inserted requires additional components and
assembly time to form a plate. Moreover, multi-component designs
may lose their ability to retain a fastener over time due to
material failure, relaxation, or the like. Additionally,
multi-component configurations may not provide sufficient ability
to lag the plate to the vertebral body.
One additional drawback of many designs is that they add to the
overall height of the plate. It is desirable to maintain a low
profile design for many reasons, such as to minimize irritation to
surrounding tissue. For example a plate design having a high
overall height or a receptacle design that does not prevent screw
backout may cause a patient to suffer from dysphasia. Ultimately,
the screw or plate may irritate or wear through neighboring tissue.
In addition, a high height plate or unretained loose screw in the
lumbar spine may be abrasive to the aorta or vena cava. Severe
abrasion by the plate or screw in this instance may puncture the
aorta or vena cava and cause internal bleeding.
In addition, many of these plates were not designed to allow for
the locking of all of the screws, which left some of the screws
susceptible to backout caused by tiny vibrations, or micromotion.
Some methods attempted to reduce the profile of the total system by
using small parts. However, this led to the small parts falling off
and getting lost. In addition, the smaller parts are fragile and
require special instruments in order to insert or manipulate them.
In addition, because of their small size, incorrect placement
relative to the axis of the plate often causes sharp and jagged
shavings to be formed as a locking screw contacts an improperly
seated bone screw.
Prior attempts at increasing the screw purchase have resulted in
risky procedures, or an insufficient ability to adapt to movement.
Attempts and decreasing the profile of bone fixation plates have
resulted in lost parts, or insufficient purchase. A continuing need
exists for an apparatus that is able to quickly and reliably lock a
plurality of screws into place while maintaining a low profile.
SUMMARY OF THE INVENTION
The present invention relates to an apparatus for connecting a
plate to a bone. This may be desirable in order to immobilize, for
example, two cervical vertebrae. In one embodiment, the present
invention comprises at least one screw and a plate having at least
one opening. As the screw passes through the opening and is
tightened, it begins to lag the plate to the bone. When the screw
head interferes with the pate at an interference point, there is a
slight resistance force that insertion forces can easily overcome.
When the screw is advanced further, it snaps into the sliding fit
area and is allowed to move freely. Forces which can cause the
screw to back out of from the plate are preferably not strong
enough to pass the screw head past the interference section. In
some embodiments, it may be desirable to use a set screw to aid in
preventing backout. Alternately, a clamp applied to the head of the
screw to prevent rotation may be desired.
In one embodiment, the present invention comprises an apparatus for
fixing a plate to bony material, comprising at least one opening
having a spherical curvature. Also included is at least one
fastener having a head that interferes with the spherical curvature
at an interference point. In this embodiment, the head is capable
of engaging with and passing the interference point to communicate
with the spherical curvature.
In some embodiments, the spherical curvature includes at least one
engagement area and at least one relief area. The tangents to the
spherical curvature preferably intersect to form an angle.
Preferably, the angle of intersection of the tangents is between
about 1 and about 5 degrees. More preferably, the angle of
intersection of the tangents is between about 1 and about 3
degrees.
It is desirable to limit the relief areas in some embodiments to
prevent a screw from passing through the interference point.
Accordingly, it is preferred that the relief area comprises less
than about 40% of the circumference of the spherical curvature.
More preferably, the relief area comprises less than about 30% of
the circumference of the spherical curvature. In some embodiments,
it may be desirable to provide an additional opening that is
configured and dimensioned to increase the magnitude of
interference at the interference point.
In another embodiment, the present invention comprises an apparatus
for stabilizing at least two bony structures, comprising a plate
where more than one aperture is configured and adapted to include
an interference area. The interference area is integrally formed in
the plate to prevent a fastener from backing out of the
interference area.
In this embodiment, a fastener, such as a screw, is capable of
engaging with and passing through the interference area. The
interference area is part of spherical curvature, which has at
least one engagement area and at least one relief area.
Preferably, the tangents to the spherical curvature intersect. It
is desirable to have the angle of intersection of the tangents
between about 1 and 5 degrees. In some embodiments, it is also
preferable to include another opening that is selectively
positioned to increase the magnitude of interference at the
interference point. The opening may be configured and adapted such
that it is able to pass a wedge shaped screw.
In another embodiment, the present invention comprises an apparatus
for fixing a plate to bony material consisting essentially of at
least one opening having a spherical curvature. At least one
fastener having a head capable of engaging with and passing through
an interference point of the spherical curvature is also included.
In this embodiment, the fastener is prevented from backing out of
the opening by the interference point. In this embodiment, the
tangents to the spherical curvature intersect. As described above,
another opening may be selectively positioned to increase the
magnitude of interference at the interference point.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing one embodiment of the bone fixation
plate according to the present invention;
FIG. 2 is a diagram showing a side view of exemplary openings
according to the present invention;
FIG. 3A is a diagram showing one embodiment of the spring loaded
plate according to the present invention;
FIG. 3B is a diagram showing an exemplary ramped surface included
in the spring loaded plate of FIG. 3A;
FIGS. 4A and 4B are diagrams showing an exemplary embodiment of a
set screw according to the present invention;
FIGS. 5A and 5B are diagrams showing an exemplary embodiment of a
bone screw according to the present invention;
FIG. 6 is a diagram showing another embodiment of the bone fixation
plate according to the present invention;
FIG. 7 is a diagram showing one embodiment of the spherical
curvature according to the present invention;
FIG. 8 is a diagram showing the forces exerted by the screws
according to the embodiment shown in FIG. 6;
FIG. 9 is a diagram showing another embodiment of the bone fixation
plate according to the present invention;
FIGS. 10A and 10B are illustrations of additional embodiments of
bone fixation plates of the present invention;
FIG. 11 is a diagram showing a one embodiment of the bone fixation
plate according to the present invention;
FIG. 12 is a diagram showing a drill guide in communication with a
bone fixation plate of the present invention;
FIG. 13 is a magnified view of a drill guide in communication with
a bone fixation plate of the present invention;
FIG. 14 is a side view of a drill guide in communication with a
bone fixation plate of the present invention;
FIGS. 15A-C are diagrams showing an exemplary embodiment of a rigid
bone screw according to the present invention; and
FIG. 16 is an illustration of one embodiment of a drill guide
capable of rotating about an axis of a receptacle or depression
formed in the plate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a bone fixation plate that
minimizes the problems associated with prior bone fixation plates
while maintaining a small profile. In one embodiment, as a screw is
tightened, it will begin to lag the plate to the bone. When the
screw head interferes with the plate at an interference point, a
slight resistance is generated. The insertion forces can easily
overcome this resistance. When the screw is advanced further, it
snaps into a sliding fit area and is allowed to move freely. The
forces which can cause the screw to back out from the plate are
preferably not strong enough to pass the screw head back past the
interference section. It may be desirable to include a set screw to
prevent backout of the screws due to micromotion. In other
embodiments, the head of the screw may be clamped to prevent
rotation, when such a restriction on the movement of the screw is
desirable.
The present invention provides a locking mechanism that allows one
or more bone screws used for attaching a plate to vertebrae to be
easily and reliably locked in place at the same time by a single
operation. When fully installed, the locking mechanism has a low
profile and maintains its ability to prevent breakout of screws due
to micromotion. The present invention may be used on the anterior
or posterior of the vertebrae. Although the present invention is
described with respect to two bone fixation vertebrae, it will be
understood that the following embodiments are capable of being used
with any number of vertebra, in any spinal location.
Turning now to the drawings, FIG. 1 shows one embodiment of a bone
fixation plate 101 according to the present invention. The plate
may be secured to two vertebrae in order to maintain the vertebrae
integrally with respect to one another in a desired orientation and
at a desired spacing from one another. Plate 101 includes two
fastening devices, such as screws 103-105 or the like, which are
operatively communicable with spring loaded plates 107-109. The
plate also includes four openings 111-117, through which screws
(not shown) may be used to fasten the plate 101 to the
vertebrae.
The plate 101 and the screws may be comprised of any material, such
as a metal, alloy, or any combination of the two. Preferably, the
material used to construct the plate and the screws allows the
plate 101 to maintain its structural integrity while allowing for a
desired amount of resiliency. Furthermore, the material used is
preferably bio-compatible and capable of withstanding the
conditions of a body over a desired period of time. In some
embodiments, this is achieved by manufacturing the plate 101 and
screws using metals such as titanium or stainless steel. Titanium
has sufficient ductility to permit a desired amount of curving of
the plate 101 to conform to the shape of the vertebrae, yet has the
strength to maintain its structural integrity.
In the FIG. 1 embodiment, the bone fixation plate 101 comprises a
center portion 119 and two distal portions 121-123. Each distal
portion 121-123 may be attached to a different vertebra using
fasteners, such as screws, that pass through openings 111-117.
Because distal portions 121-123 are similar, only the operation of
distal portion 121 is described in detail.
FIG. 2 is a diagram showing a side view of openings 111 and 113. In
one embodiment, each opening has a substantially circular shape, as
shown in FIG. 1. In this embodiment, the inner portion of openings
111-113 have substantially spherical curvatures. Accordingly, the
radius of the inner portion of openings 111-113 decrease in
diameter from the top 201 of the openings, to the bottom 203 of the
openings. Preferably, the spherical curvature of the openings
111-113 may accommodate a screw having a spherical head. However,
the present invention is not limited to spherical curvatures or
spherical heads. In other embodiments, any complementary head and
receptacle may be used. Preferably, the complementary head and
receptacle are capable of preventing the breakout of the screw.
As shown in FIG. 2, the openings 111-113 are not continuous. It is
desirable that the openings 111-113 comprise only a portion of the
circumference of the spherical curvature. In one embodiment, the
remaining portion 205 of the circumference of the spherical
curvature of the openings 111-113 is provided by spring loaded
plate 107, shown in FIG. 1. The portion of the circumference of the
spherical curvature that is completed by spring loaded plate 107
may be varied as desired, for example, according to the amount of
resistance that is desired by the spring loaded plate 107. In one
embodiment, the openings 111-113 comprise at least 60 percent or
more of the total circumference of the spherical curvature. In
another embodiment, the openings 111-113 comprise at least 70
percent or more of the total circumference of the spherical
curvature. In yet another embodiment, the openings 111-113 comprise
at least 80 percent or more of the total circumference of the
spherical curvature.
FIG. 3A is a diagram showing one embodiment of the spring loaded
plate 107. In this embodiment, the spring loaded plate 107 includes
arm 301. When a force causes arm 301 to be deflected towards the
body 303 of the spring loaded plate 107, potential energy is stored
in the arm 301. This potential energy causes the arm 307 to
generate spring-like forces that have a tendency to force it away
from the body 303, and back to its natural resting position shown
in FIG. 3A. When the deflection force is removed, the potential
energy is converted to kinetic energy, and forces the body 303 away
from the arm 307. In other embodiments, the spring loaded plate 107
does not have to have a free cantilever load such as the arm 301
shown in FIG. 3A. For example, it may be desirable to use a loop,
or the like, to resist movement of the spring loaded plate 107.
The inner portion of plate 107 preferably comprises a ramped
surface 305. In one embodiment, the ramped surface 305 is
selectively engageable with screw 103, shown in FIG. 1. When the
screw 103, is engaged by the ramped surface shown in FIG. 3B,
outward forces are generated on the screw, preventing it from
backing out. As the angle of the ramped surface increases, the
forces that are exerted on the screw 103 increase. Thus, the angle
of the ramped surface may be chosen based on the amount of force
that is desired to keep the screw 103 from backing out.
In one embodiment, the angle of the ramp is between about 5 and 50
degrees. In another embodiment, the angle of the ramp is between
about 10 and about 30 degrees. In yet another embodiment, the angle
of the ramp is between about 15 and 25 degrees.
The spring loaded plate 107 comprises two spherical curvatures 307
and 309. Spherical curvatures 307 and 309 complete the spherical
curvatures of openings 111 and 113. Each curvature 307-309
comprises a spherical curvature having a radius that decreases from
top to bottom, as discussed with respect to the curvatures of
openings 111 and 113. The spherical curvatures 307-309 may comprise
any desired percentage of the circumference of the total spherical
curvature. In one embodiment, each curvature 307-309 may comprise
20 percent or less of the total circumference of the spherical
curvature. In another embodiment, each curvature 307-309 may
comprise 30 percent or less of the total circumference of the
spherical curvature. In yet another embodiment, each curvature
307-309 may comprise 40 percent or less of the total circumference
of the spherical curvature.
Spring loaded plate 107 also includes two edges 311 and 313, shown
in FIG. 3A. Each edge is preferably configured and dimensioned to
be engageable with a depression 125 in plate 101. In one
embodiment, the spring loaded plate 107 is positioned within the
depression 125. Depression 125 is configured and dimensioned such
that there is sufficient space for plate 107 to move between its
compressed and relaxed states, described with respect to FIGS. 3A
and 3B. In one embodiment, plate 107 is prevented from horizontally
exiting depression 125 by the protrusion formed by openings
111-113.
In one embodiment, shown in FIGS. 4A and 4B, the screw 103 may have
an angled head 401. It may be desirable for screw 103 to have
threads along its elongate shaft 403. In order to aid in tightening
screw 103, it preferably includes a projection 405 with a curved
surface to aid in gripping the screw. The length of the elongate
shaft may be varied as desired. In one embodiment, the length of
the elongate shaft is about 5 mm or less. In another embodiment,
the length of the elongate shaft is about 3 mm or less. In yet
another embodiment, the length of the elongate shaft is about 1 mm
or less.
FIGS. 5A and 5B are diagrams showing one embodiment of the screw
that is used to connect plate 101 to vertebrae. Screw 501
preferably has a spherical head 503 that is selectively engageable
with the spherical curvature. An elongate shaft 505 is connected to
the spherical head 503 to allow it to penetrate bony tissue of the
vertebrae. Preferably, the elongate shaft 505 includes threads that
aid in fixing the plate 101 to a vertebra. As shown in FIG. 5B, it
is desirable to have a hexagonal projection 507 to aid in gripping
the screw.
The length of the elongate shaft 505 may be varied as desired. In
one embodiment, the length of the elongate shaft is about 20 mm or
less. In another embodiment, the length of the elongate shaft is
about 10 mm or less. In yet another embodiment, the length of the
elongate shaft is about 5 mm or less.
In one embodiment, screw 103 is inserted into a receptacle in
depression 125. It is desirable to have a threaded receptacle such
that the screw is capable of being fixed to the plate 101. The
screw 103 also passes over plate 107, and prevents it from
vertically exiting depression 125. The placement of the screw
receptacle is preferably chosen such that it is engageable with the
ramped surface 305 of plate 107 when the plate is in its relaxed
state, with its arm 301 extended.
Preferably, two screws 501 are inserted into openings 111 and 113.
As the screws 501 are tightened, they will begin to lag the plate
101 to the bone. When the screw head 503 interferes with plate 107,
it forces it to move towards the center of the plate 101. As the
screws 501 are advanced further, the plate 107 forces its way back
into its relaxed state. This causes the spherical curvatures
307-309 to form a complete spherical curvature around the screw
head 503. When plate 107 is in its relaxed state, it prevents screw
501 from backing out. It may be desirable to tighten screw 103,
such that plate 107 remains fixed in its relaxed state. In this
manner, the screw 501 is prevented from backing out.
Screws 501 may be screwed into bony tissue at any desired angle. In
other words, screw 501 does not have to be inserted perpendicular
to the plate 101. The spherical properties of the head of the screw
503 and the spherical curvature of the openings 111-117 are
preferably sufficient to prevent the screw from backing out. Thus,
the largest diameter of the head of the screw is larger than the
diameter of the narrowest portion of the opening in the top our
outer side of the plate through which the screw head is placed. The
interference difference between the fastener head diameter and the
outer narrow opening may be describe in different ways depending on
the size of the plate, openings, and fastener heads being used. For
example the interference difference between the fastener head and
the narrowest opening may be about 0.01 mm or greater, about 0.03
mm or greater, or about 0.10 or greater, or even about 0.20 mm or
greater. Preferably, however, in each instance the interference is
less than about 2 mm.
Alternatively, the interference between the fastener head and the
narrow outer opening may be described relative to the outer
diameter of the fastener head itself. For example, the interference
may be about 0.5% or greater of the diameter of the fastener head,
about 5% or greater of the diameter of the fastener head, or even
about 10% or greater of the outer diameter of the fastener head.
Preferably, however, in each instance the interference is less than
about 40% of the outer diameter of the fastener head.
While openings 111-117 prevent the screws 501 from backing out,
they do allow it to rotate freely within the spherical curvature.
One advantage of allowing the screw 501 to rotate freely is that
the bone fixation plate according to the present invention is able
to accommodate for movements in the vertebrae or accommodate for
compression of the bone grafts that are placed between vertebrae.
Another advantage of allowing the screws to be inserted at any
angle is that it allows relatively close spacing of the screws,
without the risk of interference with one another.
FIG. 6 shows another embodiment of the present invention. As shown
in FIG. 6, an exemplary bone fixation plate according to the
present invention comprises four openings 601-607. In one
embodiment, openings 601-603 are connected to one vertebra, and
openings 605-607 are connected to a second vertebra. Also included
are two additional openings 609-611, which are located at a desired
point between points 601-603 and 605-607, respectively. One
advantage of the FIG. 6 embodiment is that a screw does not have to
be inserted into opening 609 until after screws are inserted into
openings 601-603. Thus, opening 609 serves as a window for a
surgeon to view the vertebra, or space between adjacent vertebrae.
This is often desirable to the surgeon.
Because all of the corresponding openings are similar, only
openings 601-603 and 609 are described in detail. In one
embodiment, openings 601 and 603 are spherical curvatures having
the same properties discussed with respect to FIGS. 1-5. Thus, a
complete description of the openings 601-603 is not repeated. The
openings are substantially similar in size, shape, and diameter to
the openings 111-117 described with respect to FIG. 1. There are
some differences between the openings shown in FIG. 6 embodiment
and the openings shown in the FIG. 1 embodiment, which are
discussed below.
In one embodiment, the spherical curvature of openings 601-603 is
substantially circular. The opening 601-603 comprises the majority
of the circumference of the spherical curvature for the screw 613.
This is in contrast to the spherical curvatures described with
reference to FIGS. 1-5, which were formed by both the openings and
the spring loaded plate 107. Therefore, the spherical curvature of
each opening 601-603 houses substantially entire head of screw 613.
In one embodiment, screw 613 is substantially similar to screw 501,
discussed with reference to FIGS. 5A and 5B.
In one embodiment, the spherical curvature of the opening 601-603
comprises 90% or more of the total circumference of the curvature.
In another embodiment, the spherical curvature of the opening
601-603 comprises 95% or more of the total circumference of the
curvature. In yet another embodiment, the spherical curvature of
the opening 601-603 comprises 99% or more of the total
circumference of the curvature.
In one embodiment, opening 609 is selectively positioned between
openings 601-603. Opening 609 preferably allows a screw 615 to
pass, which increases the interference between the spherical
curvature of the opening 601-603 and the head of the screw 613. As
shown in FIG. 6, the placement of the openings 609-611 may be
varied. In one embodiment, the opening may be positioned such that
it positioned directly in between the openings or slightly higher
than the openings. However, in another embodiment the opening
609-611 may be placed at a desired distance above the openings.
In order to use screw 615 to cause tighten the openings 601-603
around the head of screw 613, openings 601-603 comprise a fixed
portion and a flexible portion 617. In one embodiment, flexible
portion 617 is formed by a discontinuity that is formed in openings
601-603 and opening 609. When the flexible portion 617 of the
opening is pushed against the screw 615, increased interference
results. One advantage of the increased interference is that
backout of the screw 615 is prevented.
The discontinuity 619 should be large enough that it allows flexure
of portion 617 of the spherical curvature, while allowing the
spherical curvature to maintain its structural integrity and
provide a sufficient contact area for the head of the screw 613. In
one embodiment, the discontinuity 619 shown in FIG. 6 comprises a
small portion of the total circumference of the opening 601-603.
The discontinuity 619 may be vertical, or it may be configured and
dimensioned at a desired angle.
In one embodiment, the discontinuity 619 comprises about 5% or less
of the total circumference of the curvature. In another embodiment,
the discontinuity comprises about 3% of less of the total
circumference of the curvature. In yet another embodiment, the
discontinuity comprises about 1% or less of the total circumference
of the curvature.
In the FIG. 6 embodiment, tangents to the curvature of opposing
points along the spherical curvature intersect. This is in contrast
to typical spherical curvatures that have been used for bone
fixation plates, where the tangents to the curvature of opposing
points do not intersect. One advantage of having the tangents to
the curvature intersect is that the spherical curvature generates
an interference area. As the head of the screw being screwed into
place, a sufficient amount of force may be applied to force the
head of the screw to contract slightly. As the screw is continues
into the bone, the head of the screw is able to pass through the
interference area. Once the head of the screw passes through the
interference area it fits into the spherical curvature. It is
desirable that forces that force the screw to backout are not
strong enough to force the screw back through the interference
area. This resistance of the interference area may be modified by
changing its curvature.
In one embodiment, tangents to spherical curvature intersect to
form an angle, as shown in FIG. 7. This angle is preferably between
about 1 and about 10 degrees. In another embodiment, the angle
between the tangents is between about 1 and about 5 degrees. In yet
another embodiment, the angle between the tangents is between about
1 and about 3 degrees.
In one embodiment, opening 609 is substantially similar to openings
601-603. That is, it has a substantially spherical curvature. In
other embodiments, however, opening 609 may not have a
substantially spherical curvature. The curvature may be shaped to
receive a screw 615 having a flat head. However, other types of
screws may be used. In embodiments where screw 615 has threads, the
receptacle may be configured to receive the threads in order to
prevent screw 615 from backing out. To allow an instrument to grip
the screw, a hexagonal depression may be configured on the head of
the screw. However, in other embodiments it may be desirable to
have a curved protrusion to aid in gripping the screw 615.
The diameter of opening 609 is preferably smaller than the diameter
of openings 601-603. The diameter of opening 609 may be smaller
than the diameter of openings 601-603 because the screw 615 that
passes through the opening does not have to pass through bony
tissue. In one embodiment, opening 609 and screw 615 function to
further restrict openings 601-603 after the screw 613 has been
inserted.
Screw 615 may be screwed into bony tissue at any desired angle. In
other words, screw 615 does not have to be inserted perpendicular
to the plate. The spherical properties of the head of the screw 615
and the spherical curvature of the openings 601-603 are preferably
sufficient to prevent the screw from backing out. While openings
601-603 prevent screw 613 from backing out, they do allow it to
rotate freely. One advantage of allowing the screw 613 to rotate
freely is that the bone fixation plate according to the present
invention is able to accommodate movements in the vertebrae or
accommodate for compression of bone grafts that may be placed
between vertebrae. Another advantage of allowing the screws to be
inserted at any angle is that it allows relatively close spacing of
the screws, without the risk of interference with one another.
As described above, openings 609-611 may be placed in any desired
position. In some embodiments, it may be desirable to position the
opening 611 directly in line with openings 605-607. However, in
other embodiments it may be desirable to place the opening 609 at a
higher position than the openings 601-603. FIG. 8 is a diagram
showing exemplary forces that may be exerted on the openings
601-607 when screws are inserted into openings 609 and 611.
As shown in FIG. 8, when the opening 609 is positioned at a higher
position than openings 601-603, the screw exerts forces on parts
609-611 of the openings. Because the magnitude of interference
between the openings 601-603 and the screw 613 is not as great,
this embodiment may be preferable in applications where a
significant amount of movement or shifting of the vertebrae is
expected. The lower magnitude of interference allows the screws to
shift to accommodate these movements. However, when the opening is
positioned directly in line with openings 605-607, the screw exerts
forces on a larger portion of the openings 605-607. Because of the
increased magnitude of interference between the openings 605-607
and the screw 613, this embodiment may be desirable when it is
preferable to have the plate held in place with more force.
FIG. 9 is a diagram showing another embodiment of the present
invention. In this embodiment, the present invention comprises at
least two openings through which screws may pass. The screws used
in this embodiment are similar to the screws described with respect
to FIGS. 1-8, thus a discussion of them is not repeated. In some
embodiments, a third opening may be selectively positioned between
the at least two openings in order to prevent a screw from backing
out of the openings. Though only one set of openings is shown in
FIG. 9, a corresponding set of openings are also attached to an
adjacent vertebra. Adjacent sets of openings are preferably
connected by two elongate shafts 909 and 911.
As shown in FIG. 9, openings 901-903 comprise spherical curvatures,
as described with reference to FIGS. 1-8. As described with
reference to FIG. 7, tangents to the spherical curvature intersect
to form an angle. The angles are similar to those discussed with
reference to FIG. 7, and therefore are not repeated. In addition to
the spherical curvature described with reference to FIG. 7, the
FIG. 9 embodiment also includes alternating sections of engagement
905 and sections of relief 907. These sections of engagement 905
and relief 907 are preferably located along the top portion of the
spherical curvatures, from which the screw is inserted. One
advantage of having one or more sections of engagement and relief
is that the openings 901-903 are able to accommodate micromotion of
the screw, or in some cases, of the entire plate.
In one embodiment, the openings 901-903 comprise a single relief
section 907. This provides the advantage of allowing a screw to
adjust due to micromotion, while preventing the screw from backing
out. In this embodiment, the remainder of the openings 901-903 is a
section of engagement. The section of engagement preferably resists
the motion of the screw.
In another embodiment, more than one section of relief 907 may be
included. More than one relief section 907 may be desirable in
embodiments where micromotion may be prevalent. The sections of
relief 907 allow the angle of the screws to vary while preventing
it from backing out. However, it is undesirable to include too many
sections of relief 907. It is desirable to have more sections of
engagement 905 than sections of relief 907 because too many
sections of relief will result in the magnitude of the interference
point being reduced at different angles. Thus, in a preferred
embodiment, the openings 901 and 903 include more sections of
engagement 905 than sections of relief 907.
In one embodiment, the number of relief sections 907 included in
the openings is two or greater. In another embodiment, the number
of relief sections that are included in the openings is four or
greater. In yet another embodiment, the number of relief sections
that are included in the openings is six or greater.
In some embodiments, the portions of the opening that are relief
sections 907 and the portion of the openings that are engagement
sections 905 may be expressed as a percentage of the total
circumference of the openings 901-903. Preferably, the sections of
relief comprise about 50% or less of the circumference of the
openings. More preferably, the sections of relief comprise about
40% or less, and most preferably the sections of relief comprise
about 30% or less of the circumference of the openings.
In one embodiment, a third opening 913 may be placed between
openings 901-903. A set screw may be placed in opening 913 in
increase the interference of the openings 901-903 against the head
of the screw. In order to allow the screw to increase the
interference between the openings 901-903 and the screw, a wedge
shaped depression 915 may be configured and dimensioned in the
plate. The FIG. 9 embodiment provides the advantage of minimizing
the profile of the bone fixation plate while increasing its ability
to accommodate for micromotion.
In this embodiment, as a screw is tightened, it will begin to lag
the plate to the bone. When the screw head interferes with the
spherical curvature at an interference point, a small amount of
resistance is generated. The interference, and resulting
resistance, are caused by the angle of intersection of the tangents
to the spherical curvature. As described above, the spherical
curvature has tangents that intersect. The interference forces are
easily overcome by the screw head. When the screw advances further,
it snaps into the spherical curvature and is allowed to move
freely. The forces which cause the screw to back out from the plate
are preferably not strong enough to pass the screw head back past
the interference section. To further assure that the screw head
does not pass back past the interference section, the set screw
described above may be employed.
Referring now to FIGS. 11-14, the plate of the present invention
may be configured to aid in the insertion of bone screws. For
example, FIG. 11 illustrates that the plate may have one or more
openings 1101 that are capable of securely receiving a drill guide.
For example, the openings may be configured with threads that
engage with a threaded tip of the drill guide. In addition, the
plate may also have one or more recesses, pivot points, depth
stops, or areas of removed material in the top surface of the plate
that help align the drill guide opening over the holes of the
plate. The drill guide may have a rotating barrel that rotates
along an axis that extends through the recess of the plate. In one
embodiment, a portion of the drill guide can be aligned with and
contact the recess while providing a base on which the barrel can
rotate. Alternatively, as shown in FIG. 16, a portion of the barrel
itself may reside in the recess of the plate upon which the drill
guide may be rotatably disposed.
As shown in FIG. 13, the barrel may have a drill bore extending
through its length. When the drill guide is properly aligned with
the recess and opening, the barrel may be rotated to a first
position such that the drill bore is aligned over a hole in the
plate where a bone fastener will be placed. Preferably, the bore is
configured so that its axis passes through the spherical opening in
the plate. The portion of bone beneath the plate may then either be
prepared for receiving a fastener by drilling a pilot hole in the
bone, or alternatively a fastener may be placed directly into bone.
To further ensure that the fastener is inserted at a proper angle,
it may be inserted through the bore.
Once a first fastener has been inserted into a first hole of the
plate, the barrel may be rotated such that it is aligned over a
second hole in the plate, thereby allowing a second fastener to be
inserted without having to reposition the entire drill guide. As
shown in FIGS. 11-14, a plurality of guide holes and recesses may
be provided in the plate. In one embodiment, one recess and guide
hole may be used to insert fasteners into two bone screw holes.
As discussed above, once a fastener head has passed the
interference area it may freely swivel or rotate to accommodate
different angles or to allow for reabsorption of graft material
over time. As graft material or bone is reabsorbed by the body,
loading previously borne by the bone may be transferred instead to
the plate. Thus, in may be preferable in some circumstances to have
one or more fastener, more preferably two or more fasteners remain
substantially free to swivel or rotate to account for dimensional
changes in the bone that may occur after insertion of the
plate.
In some cases, however, it may be desirable to rigidly fix the
angle of the fastener relative to the plate once it is deployed.
While some devices have been developed in the past to help resist
or prevent micromovement of a plate relative to a fastener or to
the bone that the plate contacts, past designs have either lacked
the ability to be inserted at varying angles or have required
complex designs or additional components in order to achieve
multi-angel variability.
One example is described in U.S. Pat. No. 4,484,570, which is
incorporated herein in its entirety. In particular, this reference
discusses that reabsorption of the bone may take place at a portion
of the contact surface between the bone and the plate. Over time,
this reabsorption can cause open gaps to be formed, which may
eventually become large enough that varying loads acting on the
bone can cause undesirable micromovement between the plate, bone,
and fasteners. This reference addresses this issue by describing a
fastener having a head configured with a generally conical outer
surface and having one or more slots. The fastener head further has
a clearance hole or receptacle in which an expanding set screw may
be inserted to splay or direct portions of the slotted head
radially outwards. The interior surface of the fastener head is
also generally conical and corresponds to a conical outer surface
of the set screw. Thus, as the set screw is driven further into the
clearance hole or receptacle, the interaction between the two
conical surfaces applies progressively greater amounts of locking
force. As mentioned above, one disadvantage to the locking fastener
system described in the '570 patent is that it is not capable of
permitting adjustability of the fastener with respect to the
plate.
Another example is found in U.S. Pat. No. 6,235,033, which also is
incorporated herein in its entirety. In particular, the '033 patent
purports to achieve multi-angle variability of a plate design based
substantially upon the addition of a c-ring to the design described
in the '570 patent. In particular, the '033 patent likewise teaches
to use a fastener having a slotted head with a generally conical
outer surface. The conical surface of the fastener can be connected
to a c-ring that resides in the opening of the plate through which
the fastener is inserted. The outer surface of the c-ring slidingly
engages with the spherical surface of the hole in the plate to
provide variation in the angle of the fastener. When desired, an
expansion screw may be deployed in a receptacle formed in the
fastener head so that the outer surface of the fastener head apply
outward pressure against the c-ring. Eventually, the c-ring expands
sufficiently to apply pressure against the opening in the plate
that locks the fastener relative to the plate. One disadvantage to
this multi-angle locking system, however, is that it requires that
the plate be assembled with a c-ring in each opening or hole
through which a fastener will be placed.
While any of the various methods and techniques described in these
references for having the fastener head capable of applying an
outward force may be used, the present invention also relates to an
improved way to achieve multi-angle variability while preserving
simplicity of design. Rather than using a complex, multi-piece
plate construction or sacrificing the ability of the fastener to
have variable angles relative to the plate, the present invention
contemplates forming the outer surface of a slotted fastener head
to have a curved or spherical shape corresponding generally to the
curvature of a portion of the plate holes in which the fastener
will be placed. Thus, the fastener may be inserted into a plate
hole at a variety of angles and be selectively locked in position
without the use of a c-ring, bushing, or the like to aid in
providing multi-angle variability. Once the fastener is in its
desired position, a set screw may be inserted into a receptacle in
the fastener head to rigidly hold the fastener in a fixed position
relative to the plate.
The outer surface of the curved fastener head may be textured to
provide increased locking forces. For example, a portion of the
outer surface of the fastener head may be configured with circular
grooves that help hold the fastener in place as the slotted head is
expanded outward against the inner surface of the plate hole.
Likewise, the outer surface of the fastener may be roughened to
provide increased resistance to slippage between the fastener head
and the plate when in a rigid position.
FIGS. 15A-C illustrate one example of a fastener of the present
invention that is capable of selectively providing the ability to
swivel or move and to hold a fixed position. In particular, the
fastener head has an outer surface that is generally spherical in
shape, thereby allowing it to rotate or swivel once past the
interference area. The fastener head also has a plurality of slots
or cuts in the head that permit the head to expand or compress. The
fastener head also may have an interior space that is capable of
securely receiving a second fastener, such as a set screw, a cam,
or the like. As the second fastener is inserted into this interior
space, its causes the diameter of the first fastener head to
increase and press against the inner wall of the bone screw opening
in the plate, thereby locking it in place. As stated above, the
outer surface of the first fastener head may be textured to further
increase the ability to rigidly hold the fastener in place.
The inner space of the first fastener and outer shape of the second
fastener may have different configurations to create and apply a
locking force. For example, in one embodiment the set screw,
interior space, or both may have a generally conical shape that
progressively applies greater outward forces as the set screw or
second fastener is inserted. Likewise, the interior space, second
fastener head, or both may be generally cylindrical with the
diameter of the second fastener being greater than the inner
diameter of the interior space.
Thus, in accordance with the present invention, the bone fixation
plates and components described with reference to FIGS. 1-15 may be
secured to vertebrae and other bony material in a manner that
prevents the screws from working loose when subject to vibration.
Furthermore, the embodiments described above prevent the backing
out of screws while minimizing the profile of the bone fixation
plate. Retaining features, provided near each opening through which
a screw may pass, is moveable between relaxed and flexed positions.
Another advantage of the present invention is that screws that
fasten the plate to the bony tissue may be oriented at a variety of
non-perpendicular angles with respect to the plate, which allows a
relatively close spacing of fasteners without the risk of fasteners
interfering with one another.
Although the present invention has been described with respect to
several embodiments, it will be understood by those skilled in the
art that the present invention is capable of alternate embodiments
within the spirit of the appended claims. For instance, while the
embodiments described herein refer to a plate useful for the
cervical region of the spine, skilled artisans would understand
that the plate design described herein may also be used in other
regions of the spine or even for fixation of other bones in other
parts of the body. Thus, the invention is not limited only to
treating the cervical spine.
* * * * *